Many applications have a need for a reliable power source. For example, telecommunication systems typically demand a DC power supply of high availability. Battery plants have been developed to satisfy these DC power demands. However, many applications also require AC power supplies of high availability. For example, modern telecommunication systems also rely on AC powered equipment to perform various monitoring and control functions. Such AC powered equipment is often as critical to successful operation of the telecommunication system as the DC powered equipment mentioned above. Accordingly, highly reliable AC power sources are needed in such applications.
Understandably, telecommunication companies are often reluctant to bring commercial AC power into intimate contact with critical loads. This reluctance is rooted in several issues including the risk of power outages and the risk of transients that can potentially damage parts of the telecommunication equipment, both of which are associated with such commercial power systems. Telecommunication companies, thus, often employ uninterruptible power supplies to provide their AC supply needs.
However, standard uninterruptible power supply (UPS) systems require a major investment in resources. For example, such systems often require purchase of a separate battery plant to avoid compromising the prime DC source. A more cost effective approach to providing a UPS system is to employ a redundant inverter system operating off the existing telecommunication battery that is entirely isolated from utility power. However, the inverters in these redundant inverter systems have some unique requirements. For example, while N+1 redundancy (i.e., including at least one more power supply than is needed to supply the load) can be used to achieve high availability DC power in a fairly straightforward manner, applying N+1 redundancy to AC power supplies is more complicated. Specifically, when connecting multiple inverters in parallel to achieve N+1 redundancy in the AC context, it is necessary to match both the phase and the amplitude of the parallel inverters in order to achieve equal load sharing. A failure to properly load share results in undesirable cross conduction current flowing between the inverters. In the prior art, phase and amplitude matching is performed with common synchronization and load share circuitry which couples the parallel inverters together. Unfortunately, such common circuitry inherently compromises the redundancy advantage by rendering the system susceptible to "single fault" failures in the common circuits that can disrupt the AC power supply. In fact, failure in the common synchronization or signaling circuits can bring down the whole inverter system.
Prior art systems that employ common load share circuitry between parallel inverters typically utilize an isolation circuit and a phase locked loop to bring an inverter voltage of an inverter being added to the system into phase alignment with the AC bus voltage prior to actually connecting the new inverter to the bus. Once phase matching is achieved, the isolator, (typically implemented by a relay), connects the inverter to the bus without significantly disrupting the voltage.